The present disclosure relates to fluid flow control systems, such as intravenous infusion pumps, and more particularly to feedback control infusion pumps with flow sensing, volume sensing, variable pressure control, and variable flow resistance. In particular, the present disclosure relates to a method and apparatus for extracting enhanced information from an in-line fluid flow sensor by imposing an abrupt flow rate change.
A conventional large volume infusion pump is typically equipped with a motor that, in connection with a mechanical assembly and through the interface of a fluid barrier, pushes a small amount of fluid per motor “step.” The mechanism might be a cam, a leadscrew, or other such assembly. The fluid barrier might be a syringe, an extruded tube, a molded cassette, or other such device that separates the pumping mechanism from the fluid in question. In each case, the fluid movement is determined by a certain number of motor steps over time.
At slow flow rates, the motor steps are relatively infrequent with long dwell periods. At high flow rates, the motor and mechanism are run at their maximal capacity until one element has reached its engineering limit. The flow rate is inherently pulsatile, yet this pulsatile nature is less significant at higher flow rates where the natural compliance of the outlet of the pumps serves to dampen the pulses into more or less a continuous stream of fluid.
The motors used conventionally are inherently powerful enough to overcome significant forces and resistances, so they are capable of generating significant pumping forces. This forceful pumping is an artifact and has no desirable clinical effect. The sensing mechanisms commonly used are pressure based and are made with indirect contact with the fluid to be pumped. In most cases, the fluid barrier, such as an extruded tube, exerts far more force than the internal fluid pressures. The result is a lack of sensitivity to pressure changes and a lack of feedback as to the actual conditions of fluid flow. It is common for conventional pumps to operate indefinitely without recognizing that the actual fluid flow rate is far below the targeted level.
Conventional motor driven pumps are notoriously inefficient with respect to external power consumption. For devices that have a high requirement for portability, this power inefficiency translates into unreliable operation.
Prior to the use of pumps, most infusions were done by the adjustment of a gravity-based pressure (e.g., by adjusting the height of a liquid container) and the adjustment of inline resistance (e.g., by moving the position of a roller clamp), both in response to an inline flow sensing method (e.g., performed by a user counting drops into an air chamber). Although this prior art method was labor intensive and had a limited rate range, it offered some significant advantages over the subsequent “advances” in technology. First, the use of gravity head heights for a delivery pressure was energy efficient. No external power supply was required. Second, the pressure was low, so the dangers of high-pressure infusions were avoided. Third, the gravity infusions could be augmented with a low cost and readily available pressure cuff, supplementing the fluid flow to rates well above those possible by an instrumented “pump” line. Forth, a gravity administration was not capable of infusing large amounts of air into the output line, because the hydrostatic pressure goes to zero as the fluid source empties.
An ideal infusion system will combine the meritorious aspects of a conventional “gravity” infusion with the benefits of a controlled intravenous infusion pump. In each aspect, this disclosure takes the desired principles of a gravity infusion and reduces the dependence upon skilled labor and extends the range and precision of fluid flow control and provides advanced information management capabilities.
An ideal embodiment of an infusion device would be one with continuous flow, wide flow rate range, high energy efficiency, accuracy of volume delivered over time, minimal operating pressures, maximum sensitivity to external conditions, freedom from false alarms for air-in-line, simplicity, low cost, intuitive operation, automated information exchange, safety, and reliability.
Certain infusions have historically been managed by air pressure delivery systems, most commonly found in the operating room and in emergency situations. Prior art attempts have been made to determine the flow rate via pressure monitoring and control. For example, U.S. Pat. No. 5,207,645 to Ross et al. discloses pressurizing an IV bag and monitoring pressure to infer flow rates. However, the prior art systems lack independent flow sensing, and, therefore, do not offer enough information to provide accurate and safe infusions.
Under the best of circumstances, there is not enough information in the pressure signal alone to provide the accuracy needed for intravenous infusion therapy. Furthermore, there are a number of likely failure modes that would go undetected using the pressure signal alone. An infusion pump must be able to respond to events in a relevant time frame. International standards suggest that a maximum period of 20 seconds can lapse before fluid delivery is considered “non-continuous.” As an example, for an infusion of 10 ml/h, the system would want to resolve 20 seconds of flow, which corresponds to 0.056 mL. This volume represents one part in 18,000 of air volume of a 1,000 mL bladder. Temperature induced change in pressure brought about by a normal air conditioning cycle is far greater than this signal. The measurement of pressure alone is not adequate for an intravenous infusion device. No general purpose, full range, infusion devices using pressure-controlled delivery are known to be on the market.
An entire class of “passive” infusion pumps exists whereby a constant pressure is created on a fluid filled container by way of a spring, elastomeric structure, gas producing chemical equilibrium, or other means. This constant pressure fluid is fed into a high resistance output line, providing relatively stable fluid flow.
In typical pressure based flow control products, a relatively high pressure pushes fluid into a known, high, and fixed resistance, providing a constant flow rate with good immunity from changes in external conditions. It is a purpose of our prior commonly owned provisional application Ser. No. 60/777,193, filed on Feb. 27, 2006, and PCT Publication Nos. WO2007/098287, WO2007/098265, and WO2007/106232 to provide a highly flexible flow control system with a very broad flow rate range, operating under minimal pressures, with a relatively low and variable resistance. The entire contents of the aforementioned provisional and PCT applications are incorporated herein by reference.
Embodiments of such devices control fluid flow based on a responsive fluid flow sensing means that forms a closed loop control by changing both the fluid driving pressure and the inline resistance. In contrast to the conventional approach to flow control wherein a user observes fluid flowing as it formed drops in an air chamber, then adjusts pressure by varying the head height of the fluid source, and then adjusts the inline resistance via a manual valve, our above-mentioned disclosures employ a flow sensing apparatus and method that automatically and accurately measures fluid flow rate, precisely adjusts the hydrostatic pressure of the fluid source, and precisely adjusts inline fluid flow resistance to achieve or maintain a target flow rate.